In construction of saddles for horse-riding, a frame which is known as a tree is used. This tree may be made of a single piece, which is generally wood, although any other suitable strong material such as composites may be used, and such a tree is known as a "fixed tree". Alternatively the tree may be made more flexible by the use of laminated strips of wood, or again other materials such as composites, which are generally reinforced by lengths of metal, such trees being known as "spring trees".
Some trees, especially conventional spring trees, include a gullet plate, which is also known as a headplate, and which comprises two arcuate plates, commonly of metal, one of which overlies the other, and which are fixed together sandwiching another component of the tree in the pommel area. These plates serve to reinforce the saddle tree in this area.
A conventional spring tree also includes two spring members, commonly strips of metal, which extend from the headplate area of the tree to the cantle area. These spring members govern flexion of the saddle in the seat area.
To complete the construction of the saddle a covering, which is known as flocking and generally consists of leather and padding, is then fitted around the saddle tree.
Conventionally, only a few sizes of tree are available, so the flocking is used to compensate for any difference in shape and size between the horse's withers and the saddle tree. Thus a considerable amount of padding is necessary in this area, which reduces the sensitivity of contact between the rider and the horse. Also, the flocking settles within a few hours of use to fit the shape and size of a particular horse at a particular time, and thereafter may not be varied. Therefore a separate saddle will be needed for each horse, and even this may not fit perfectly all the time, since the shape and size of the horse's withers may vary, for instance with the season.
After construction, the stiffness of the spring members in the seat area is also fixed. Thus the saddle will not be suitable for use by riders of widely varying weights, since each will require a different degree of tension in the springs, to protect the horse's back whilst allowing sufficient contact with the rider.